In recent years, various types of liquid crystal color displays have been developed and marketed. One of the important goals in achieving better display quality in liquid crystal displays is to improve white peak in a bright scene and contrast in a dark scene.
In one conventional method of improving contrast in a dark scene, a gain is calculated from the peak level of a video signal during predetermined intervals of time, and the output level of light is reduced and the amplitude of the video signal is extended according to the calculated gain, as disclosed in Japanese Unexamined Patent Application Nos. 102484/1994 (Tokukaihei 6-102484; published on Apr. 15, 1994), 109317/1999 (Tokukaihei 11-109317; published on Apr. 23, 1999), and 65528/1999 (Tokukaihei 11-65528; published on Mar. 9, 1999), for example. With this method, contrast can be improved in a bark scene without varying display brightness.
FIG. 13 illustrates a conventional liquid crystal display device with improved contrast in a dark scene.
The liquid crystal display device includes, for example, a peak level detecting circuit 201, a gain calculating circuit 202, an amplitude modulating circuit 203, an optical output control circuit 204, a backlight operating circuit 205, a liquid crystal driving circuit 206, a backlight 207, and a liquid crystal panel 208.
The peak level detecting circuit 201 receives a video signal and outputs a peak level. The gain calculating circuit 202 receives the output peak level from the peak level detecting circuit 201 and outputs a gain.
The output gain of the gain calculating circuit 202 is supplied to the amplitude modulating circuit 203 and the optical output control circuit 204. The amplitude modulating circuit 203, having received the output gain and a video signal, outputs an amplitude-modulated video signal, so as to drive the liquid crystal panel 208 through the liquid crystal driving circuit 206. The optical output control circuit 204, having received the output gain of the gain calculating circuit 202, outputs a backlight control signal for controlling the brightness of the backlight 207, so as to operate the backlight 207 through the backlight operating circuit 205.
Described below is one exemplary operation of the liquid crystal display device improving contrast in a dark scene.
First, in response to an input video signal, the peak level detecting circuit 201 detects a peak level during certain intervals of time, for example, such as a vertical synchronous period, and outputs the detected peak level to the gain calculating circuit 202. In response, the gain calculating circuit 202 calculates a gain value.
For example, the gain value is 0.5 when the peak value of the video is half the 100IRE value (i.e., 50IRE), and is 1.0 when the peak value of the video is 100IRE. The gain value calculated in the gain calculating circuit 202 is supplied to the amplitude modulating circuit 203 and the optical output control circuit 204. In the amplitude modulating circuit 203, the amplitude of the video signal is divided by the gain value. In the optical output control circuit 204, a normal control signal for operating the backlight 207 is multiplied by the gain value to obtain a new control signal for the backlight 207. Note that, “100IRE” is an input signal when the video is white. Accordingly, when the peak value of the video is 100IRE, it means that the peak value of the input video signal during certain intervals of time is the highest gradation level of white.
With this method, when the peak of the video signal is 50IRE for example, the amplitude of the video signal is doubled and the brightness of the backlight 207 is reduced in half. The end result of this is that the number of gradations in the video signal is doubled and contrast is improved in a dark scene, without varying display brightness.
That is, the method does not change the overall display brightness because the multiplication of the control signal of the backlight 207 by the gain value is offset by the division of the video signal by the gain value.
A magazine article “Improve Image Quality of Liquid Crystal Panel by Controlling Luminance of Light Source” (Nikkei Electronics, Nov. 15, 1999, No. 757, pp. 139–146) suggests that improved white peak luminance can be achieved by increasing the overall image brightness. The article suggests that this can be accomplished by extending the control signal for the backlight in such a manner that the output luminance of the backlight is maximized not to the level of normal operating conditions but to the level allowed by the rating of the backlight. However, a problem of this conventional method whereby white peak luminance is improved, for example, by uniformly extending the control signal for the backlight is that the output video appears brighter than the input video in a dark scene, as compared with setting the maximum luminance at normal level.
For example, when the control signal for the backlight is extended 1.2 times, the maximum luminance becomes 1.2 times greater than its normal level, thereby improving white peak luminance. However, in this case, the minimum luminance is also increased 1.2 times, causing the phenomenon of pale black display. That is, a dark scene appears bright.
Further, in this case, since the improved peak luminance is accompanied by a proportional increase in minimum luminance, the dynamic range remains unchanged from the level set by a normal luminance.
It should be noted here that a wider dynamic range can be obtained when a dark scene is displayed at a minimum luminance without the extended gain value, and when a bright scene is displayed with a peak luminance with the extended gain value.
Another drawback of the foregoing method is that the extension of the gain value that multiplies the control signal for the backlight increases the inflow of a current into the backlight, with the result that power consumption is increased. Further, improvement of white peak luminance requires a process by which display brightness is increased from its normal level.
Turning back to the prior art shown in FIG. 13, display brightness does not change, and there accordingly will be no deterioration of display quality even when contrast is improved by, for example, doubling the amplitude of a video signal and reducing the brightness of the backlight in half. This contrasts to the foregoing magazine article, which, in order to improve peak luminance, requires the output video to be brighter than the input video. That is, the improvement of peak luminance is accompanied by a change in display brightness from its original level, which may lead to deterioration of the input video in terms of display quality.
The present invention was made in view of the foregoing problems, and it is an object of the present invention to provide a display apparatus that improves peak luminance in a bright scene and suppresses pale black display in a dark scene and thereby provide a wide dynamic range without increasing power consumption.